Measuring a rotational speed, which is required in the art on many machines and systems, is physically equivalent to measuring the angular speed. It is generally known to measure the rotational speed of shafts by applying a periodic pattern known as an “encoder” (e.g. a gear) around the circumference of the shaft, which is sensed by a sensor mounted in a fixed position beside the shaft. The sensor is capable of distinguishing between tooth and tooth gap (or other periodically varying properties, such as e.g. magnetic field direction or optical transparency). The sensor then generates an output signal which has the same periodicity as the sensed pattern.
A sensor of this type outputs different signals depending on the signal processing that is present: there are approximately sinusoidal signals, which are usually generated directly from the primary sensor element, or square-wave signals, which are usually generated by comparators by the downstream signal processing. The sinusoidal signals often occur as a sine/cosine signal pair, because this combination has advantages, including with regard to detecting the direction. Any combinations of said signals are possible, so that up to four outputs (and any subset thereof) may exist: sine, cosine, square-wave in phase with sine, square-wave in phase with cosine. All of these signals are analog frequency signals, i.e. the frequency varies continuously within the interval defined by the application. The electrical output signal is a direct image of the encoder. Thus even the square-wave signals cannot be considered to be “digital”, because the discrete amplitude variable does not contain discretized information from the sensor. Depending on the implementation, there is also the possibility that the physical sensor-based process generates a signal at twice the frequency of the encoder pattern. This is the case, for instance, for certain AMR sensor elements, which have an electrical signal period that encompasses only a 180° rotation of the (encoder) magnetic field.
If sensors of the type described are used in measuring devices or control systems, then it must be taken into account that the described output signals in no way complete the measurement process: whereas most measurement systems provide output signals that are either digitally encoded or have an output value that is a direct measure of the measurand, for the sensors considered here, the measured value must first be calculated from the output waveform or pulse sequence. This applies to speed, angular-speed and rotational-speed measurements and also to angle measurements, because often absolute measurements are needed, which are obtained from the periodic signal by counting. The sine/cosine signals here have the advantage of the possibility of interpolation, but do involve a higher degree of analysis effort.
The technology presented here is used in the same way for linear position and speed measurement, but using linear rather than annular encoders.
The aforementioned incompleteness of the measurement may result in significant complexity on the receiver side of the sensor signal. Generally, a frequency counter needs to be designed, which uses the conventional methods (gate-time measurement or period-length measurement) to measure the frequency. In a control system having a microcontroller-based design, the aim is often to implement this counter without dedicated hardware by interrupting program execution whenever a level change is detected at the input to which the sensor is connected. Suitable program routines are then executed as part of the interrupt. Unfortunately, the frequent interrupts that occur at high frequencies means that a large amount of processing time for the actual control process is lost. The technique therefore definitely cannot be considered cost-free, because without the frequency counting a controller having a lower performance or lower clock frequency would have sufficed for the same application. The actual counting process is supported in many controller types directly by the integrated hardware (“capture/compare unit”); the work that remains for the program is then to determine the frequency from the counter value and to perform all the other steps.
Strict requirements for keeping any delay short and the large spread between minimum and maximum frequencies in certain applications, e.g. in the automobile, make it necessary to adjust the gate time to the measured frequency (or to swap between gate-time measurement and period-length measurement), in particular in control applications. This makes the program required more complicated and the processing time increases further.
One possible solution is to use dedicated hardware (e.g. a microcontroller or ASIC solely for the frequency measurement), but this increases cost, overall size and power consumption of the overall system.
All considerations of system architecture and costs are based on the assumption that all instrumentation and control systems today work digitally and hence each measurement result must be transferred into the memory of a processor in order to be processed further. In the simplest case, this is a microcontroller that only has an effect on a display.